1. Field of the Invention
The invention relates to an image forming apparatus in which an unfixed image is transferred to a recording material by transferring means, the unfixed image is fixed on the recording material by fixing means, and the recording material on which the image has been fixed is discharged by discharging means.
2. Related Background Art
Heretofore, in an image forming apparatus of an electrophotographic type, as shown in FIG. 10 of the accompanying drawings, an unfixed image borne on an intermediate transfer belt 102 has been transferred to a recording material P by a pair of transfer rollers 103, the unfixed image has been fixed on the recording material P by a pair of fixing rollers 105, and the recording material P on which the image has been fixed has been discharged to a discharging tray 107 outside the apparatus by a pair of discharging rollers 111.
In some cases, the pair of fixing rollers 105 suffer from a difference between the recording material conveying speed of the pair of fixing rollers 105 and the recording material conveying speed of the pair of transfer rollers 103 caused by the thermal expansion and the individual difference between the rollers or a variation in the rollers with the lapse of time. If at this time, the recording material conveying speed of the pair of fixing rollers 105 is higher than the recording material conveying speed of the pair of transfer rollers 103, the recording material P may be pulled between the pair of fixing rollers 105 and the pair of transfer rollers 103, and the disturbance or the like of the image may occur during the transfer of the image to the recording material P by the pair of transfer rollers 103, thus resulting the deterioration of the image. On the other hand, if the recording material conveying speed of the pair of fixing rollers 105 is lower than the recording material conveying speed of the pair of transfer rollers 103, the recording material P may form an excessively large loop between the pair of fixing rollers 105 and the pair of transfer rollers 103, whereby the recording material P may be strongly forced and rubbed against a conveying guide 120, and a fault image may occur.
Therefore, there has heretofore been made the following proposition which forms a predetermined loop in the recording material P between the pair of transfer rollers 103 and the pair of fixing rollers 105 to thereby prevent the recording material P from being pulled or too much flexed, and solve the deterioration of the image as mentioned above.
In Japanese Patent Application Laid-open No. H05-107966 and Japanese Patent Application Laid-open No. H07-234604, as shown in FIG. 10, a loop detecting sensor S for detecting the loop of the recording material P is provided in the conveying guide 120 portion between the pairs of fixing rollers 105 and the pair of transfer rollers 103, and in accordance with the result of this detection, the recording material conveying speed of the pair of fixing rollers 105 is changed over to a first speed lower than the recording material conveying speed of the pair of transfer rollers 103, or a second speed higher than the first speed so that a predetermined loop may be maintained.
However, in the case of the conventional art described in the above-mentioned Japanese Patent Application Laid-open No. H05-107966 and Japanese Patent Application Laid-open No. H07-234604, there is such a problem as shown below.
As shown in FIG. 10, during the time when the recording material P is being nipped and conveyed by the pair of transfer rollers 103, and the pair of fixing rollers 105, image formation can be effected while the loop of the recording material P is kept by the speed difference between the two pairs of rollers 103 and 105 based on the result of the detection by the loop detecting sensor S. However, when the trailing edge of the recording material P leaves the pair of transfer rollers 103, the loop of the recording material P is released as shown in FIGS. 11 and 12 of the accompanying drawings and therefore, the loop detecting sensor S becomes incapable of detecting the loop of the recording material P, and the actual recording material conveying speed of the pair of fixing rollers 105 for keeping the loop constant becomes undetectable. Therefore, the pair of fixing rollers 105 have nothing to do but conveying the recording material at the predetermined first speed or the second speed higher than that, and assuming that the conveying speed of the pair of discharging rollers 111 on the downstream side is equal to the conveying speed of the pair of transfer rollers 103, when the conveying speed of the pair of fixing rollers 105 was the second speed, the recording material P might become slack between the pair of fixing rollers 105 and the pair of discharging rollers 111, as shown in FIG. 11, and be rubbed against a conveying guide 122, thus resulting in a fault image. On the other hand, when the conveying speed of the pair of fixing rollers 105 was the first speed, the recording material P might become pulled between the pair of fixing rollers 105 and the pair of discharging rollers 111 and the curl of the recording material might become excessive, thus resulting in jam or fault stacking.